1. Field of the Invention
This invention relates to a heat-curable silicone rubber composition which cures into a silicone rubber having minimized compression set.
2. Prior Art
Silicone rubber compositions are well known which comprise a vinyl-containing organopolysiloxane, an organohydrogenpolysiloxane containing a hydrogen atom bonded to a silicon atom, a platinum catalyst, and an inorganic filler and which cure through addition reaction between the vinyl group and the hydrogen atom bonded to a silicon atom. Since these compositions are easy to mold and offer molded silicone rubber parts having excellent properties including heat resistance and electrical insulation, they are widely used in the application where such properties are required.
However, molded silicone rubber parts of this type are less resistant against compression and thus difficult to use as molded parts which are subject to compressive deformation for a long time, for example, rolls and connectors for automotive parts. One prior art technique for overcoming this drawback is by molding parts by press molding, injection molding or transfer molding and subjecting them to heat treatment or secondary vulcanization at a high temperature of 150.degree. to 250.degree. C., thereby improving compression resistance. This technique is based on the belief that molded silicone rubber parts can be improved in compression resistance by reducing the amount of unreacted functional groups remaining therein. This technique lowers operation efficiency because it requires the steps of placing once molded silicone rubber parts into a hot oven and heating over a long time. The final molded parts become costly.
Therefore, it is desired to have a silicone rubber exhibiting high compression resistance without the above-mentioned problem, that is, without a need for secondary vulcanization.
Japan Kokai 5-9388 or CA 2071787A discloses the addition of NC--(CH.sub.2).sub.n --CN to a silicone rubber composition to a silicone rubber having minimized compression set. However, because of the harmful nature of NC--(CH.sub.2).sub.n --CN, the resulting silicone rubber is only adapted to a limited use.
Japan Kokai 4-139258 corresponding to CA 2052410A and EP477984A discloses crosslinking between an organopolysiloxane having alkenyl groups at its side chains and an organohydrogenpolysiloxane having SiH groups at both ends thereby obtaining a silicone rubber having minimized compression set. However, this would often cause insufficient curing and give inferior physical properties of the resulting silicone rubber.